The earthquakes that occur near the plate boundary off the coast of Shikoku are classified as (a) interplate earthquakes that occur due to the slipping movement at the boundary between the subducting Philippine Sea Plate and the land plate and (b) earthquakes that occur in somewhat deeper areas within the subducting Philippine Sea Plate.

1) Interplate earthquakes caused by the subduction of the Philippine Sea Plate

Great earthquakes of this type have occurred along the Nankai Trough. The seismic ground motion from these earthquakes creates damage over a wide area, and generates tsunami that cause damage on the Pacific Ocean coast from the Kanto region to the Kyushu-Okinawa region. Some of the largest earthquakes in Japan have been of this type. An example is the 1707 Hoei earthquake (M 8.4), with a broad source region from the western part of Suruga Bay to western Shikoku. These earthquakes have recurred in the past, and there are many accounts of them in historical records.

One of these older earthquakes occurred in 684. In addition to damage caused by the seismic ground motion throughout the region, records indicate that many ships were sunk by the tsunami at Tosa, and that fields were submerged due to the crustal deformation. Later earthquakes occurred in 887, 1096 and 1099, 1361, 1498, 1605, 1707, 1854, 1944 and 1946. Therefore, great earthquakes of M 8 or so have recurred at intervals of 100-150 years along the Nankai Trough.

The area in which these great earthquakes occur is somewhat defined. Those earthquakes whose source region extends from off the coast of Shikoku to off the coast of the Kii Peninsula are called Nankai Earthquakes. Earthquakes whose source region is further east than that of Nankai Earthquakes are called Tokai earthquakes. The forthcoming Tokai Earthquake, which are the cause of great concern has a source region along the Suruga Trough. Compared to the Tokai earthquakes of the past, this source region is much smaller.

The great earthquakes along the Nankai Trough have occurred either simultaneously or in a series in adjoining source regions. Many of those that have continued in a series have started on the east side (Tokai Earthquake) and later moved to the west side (Nankai Earthquake). For example, these great earthquakes can occur over several months or years, such as the 1944 Tonankai Earthquake (M 7.9) followed two years later by the 1946 Nankai Earthquake (M 8.0). In other cases, this series can occur over a very short time, such as the December 23, 1854 Ansei Tokai Earthquake (M 8.4), followed 32 hours later, on December 24, by the Ansei Nankai Earthquake (M 8.4). In addition, two earthquakes have occurred almost simultaneously in the Tokai area and the Nankai area. An example is the 1605 Keicho Earthquake (M 7.9) and the 1707 Hoei Earthquake (M 8.4), which are thought to have occurred over the entire sea area from Tokai to Nankai.

Compared to earthquakes in other regions in Japan, these earthquakes occur at well-understood intervals. The seismic ground motion and size of the tsunami differ considerably with each occurrence, however. For example, the 1605 Keicho Earthquake generated a tsunami that struck the Pacific Ocean coast from the Kanto region to Kyushu, however, there was almost no recorded damage from seismic ground motion. Therefore, it is suggested that this was a "tsunami earthquake" ("slow earthquake" or "low-frequency earthquake") whose fault slipped more slowly than that of a normal earthquake.

A look at the crustal movement in the Shikoku region shows that the area near Cape Muroto extended in a northwest-southeast direction during the period in which the 1946 Nankai Earthquake occurred. In addition, Cape Muroto, which ordinarily subsides, uplifted about 1 meter during the earthquake. These phenomena indicate that the earthquake caused the Shikoku-side crust to greatly ride over the Pacific Ocean side one. This type of crustal movement near Cape Muroto and Cape Ashizuri accompanied by great earthquakes has occurred repeatedly along the Nankai Trough for at least the past 100 thousand years. Near Cape Muroto, in particular, there is flat terraced land (coastal terraces) where upheavals of the shallow seabed have occurred in the past. This was known to have been coastline about 125 thousand years ago, but this site has now been lifted to a height of about 200 m above sea level. In addition, the outskirts of Kochi City at the rear of Cape Muroto on the northwest side subsided about 1 meter at the maximum during the 1946 Nankai Earthquake. Damage was caused by an influx of seawater.

Since the 1946 Nankai Earthquake, compression has been observed throughout Shikoku in a northwest-southeast direction. This indicates that the accumulation of strain has begun, preparing the next Nankai earthquake, caused by the subduction of the Philippine Sea Plate.

2) Earthquakes within the subducting Philippine Sea Plate

The depth of earthquakes within the subducting Philippine Sea Plate is about 30 km near the Pacific Ocean coast in central Shikoku. To the north, for example, near the Median Tectonic Line, this depth extends to about 40 km. The depth is unclear further north. Small earthquakes periodically occur within the subducting Philippine Sea Plate beneath the Shikoku region, but large destructive earthquakes are unknown. One theory, however, holds that the 1789 earthquake in southern Tokushima Prefecture (M 7.0) occurred within the subducted Philippine Sea Plate.

In contrast, the earthquakes that occur periodically from the western part of the Seto Inland Sea to the area near the Bungo Channel appear to be related to the area of occurrence for deep earthquakes that occur below Kyushu (earthquakes within the subducted Philippine Sea Plate below Kyushu). Several earthquakes are known to have caused damage to the surrounding coastline from the western part of the Seto Inland Sea to the area near the Bungo Channel. Historical accounts indicate that destructive earthquakes of the M 7 or so occurred in 1649 (M 7.0), 1686 (M 7-7.4), and 1854 (M 7.3-7.5). An earthquake that occurred after 1868 (the beginning of Meiji era) was the 1905 Geiyo Earthquake (M 7 1/4). These earthquakes are also thought to have occurred in somewhat deeper locations within the subducted plate, though the locations have not been defined. Additionally, the 1968 earthquake at Bungo Channel (M 6.6) and the 1979 earthquake at the western part of the Seto Inland Sea (M 6.1), were events within the subducted plate at somewhat deeper locations.

Occasionally damage has been caused by large earthquakes occurring within subducted plates deep below the Kyushu region. [Refer to 9-1 (1) 2)]